Conventionally, in a fluid heating apparatus, there is disclosed a technique in which after a deposit is generated in a fuel passage, a pressure therein is increased to blow the deposit off (see Japanese Patent Application Laid-open No. 2006-183469). It is conceived that when fluid flowing through a metal tube is heated from a location outside the metal tube, molecular motion in a region near a tube wall is intensified by heat, and the molecule becomes gas. That is, as shown in FIG. 1, density of a region R2 near a tube wall R1 is lowered, and thermal conductivity of fluid is largely lowered. As a result, heat stays in fluid near the tube wall, the metal tube is locally heated and deposit is generated.
It was observed that when fluid is fuel such as hydrocarbon and alcohol, there were some molecules (hot spot) which became considerably hot. The present inventors thought that the molecules were thermally cracked and a precipitation region R3 of carbon as shown in FIG. 2 was formed in a region near the tube wall. As countermeasures, the inventors found that the density in the region near the tube wall R1 was held by increasing the pressure of the fluid as shown in FIG. 3, deterioration of thermal conductivity of fluid could be avoided, local heating could be avoided, and carbon precipitation could be suppressed.
The present invention has been achieved in view of the problem of the conventional technique and new findings, and an object of the present invention is to provide a fluid heating apparatus capable of preventing a passage from being locally heated, and suppressing carbon precipitation (generation of deposition and caulking).